1. Field of the Invention
The present invention is related to wastewater treatment filters, and in particular to a compact, high efficiency, combined biological and physical treatment filter for removing pollutants and pathogens from wastewater and other dirty water sources.
2. Background of the Invention
Decentralized wastewater treatment systems typically comprise a septic tank into which raw wastewater is introduced from one or more residences or businesses. In the septic tank or other settling means, solids are typically removed by gravity settling, and other constituents are partially removed by passive anaerobic biological treatment. When a higher level of treatment is required, the septic tank wastewater is generally directed to a biological filter or other treatment system. After treatment, the treated effluent is typically discharged into a lateral field comprising a plurality of underground parallel drainpipes or irrigation tubes that discharge the treated effluent into the underground environment through a plurality of openings on each lateral drainpipe or tube.
One type of biological filter is the biological wastewater trickling filter. Conventional trickling filters apply wastewater to the top of the trickling filter from an overhead rotary distributor. The filter is generally a fixed coarse medium comprising rocks or pieces of shaped plastic. Biological life on the surfaces of the filter media provides the primary means for wastewater treatment in such filters. The media is unsaturated to provide maximum opportunity for oxygen transfer to aerobic microbes on the media. Filters of this design are cost efficient and have wide commercial acceptance. There are several factors, however, that prevent conventional trickling filters from meeting some of the stricter established parameters for effluent discharge into the environment. Because they use a fixed media bed, these system do not rely on the superior method of backwashing to remove excess biological growth and solids from the trickling filter effluent, but instead rely on gravity sloughing, followed by settling in an external secondary clarifier. The external clarifier adds significant cost and complexity to the system that otherwise would not be present. Intermittent sand filters and recirculating sand filters are types of trickling filters that use sand instead of rocks or larger media in order to achieve a higher degree of both biological and physical treatment than conventional trickling filters. Intermittent and recirculating sand filters also rely on substantial degradation of excess solids within the media pores rather than removal in an external clarifier. Because they employ a fixed bed, intermittent and recirculating sand filters are loaded at very low rates to prevent plugging, and are also not backwashed. Finally, they do not produce final treated effluent with the quality equaling that of treated effluent from fine media filters.
A second type of biological filter is the biological aerated filter, which is used to remove nitrogen compounds and carbonaceous material from wastewater. These filters utilize submerged media to provide sites for attached fixed-film microbial growth. Biological aerated filters may be of upflow or downflow design, and are periodically backwashed. Biological aerated filters are operated in a saturated state and utilize pressurized bubble aeration to supply oxygen for aerobic microbes. Therefore, such filters require a substantial amount of energy to operate.
A third type of filter is the rapid sand filter, which is in common use in the source water treatment industry for physical filtration and in the wastewater industry as a final wastewater polishing treatment. Unlike trickling filters, rapid sand filters are backwashed through a process that fluidizes the media bed. However, rapid sand filters are run at high hydraulic loading rates and are fully saturated by maintaining a water head or pressure over the filter surface. Rapid sand filters are principally designed and operated to provide physical filtration. Biological treatment is not an objective because biological growth substantially reduces the hydraulic capacity of rapid sand filters to perform their primary function of physical filtration. Furthermore, the continuously saturated conditions in rapid sand filters are not conducive to efficient aerobic biological treatment of wastewater.
Thus, conventional solutions for aerobic biological wastewater treatment using unsaturated filter media involve fixed (that is, non-fluidizing and normally non-backwashed) media. Biological aerated filters utilize media that is periodically backwashed, but are operated in a saturated state and use bubble aeration to supply oxygen to aerobic microbes on the media. For those physical water treatment solutions that do involve backwashing, the media is saturated by pressurized water that is essentially continuously maintained above the media surface, and biological growth is viewed as a detriment to physical filtration performance rather than as a desired object of design.